Apparatus and method for virtualizing a queue pair space to minimize time-wait impacts

ABSTRACT

Apparatus and method for virtualizing a queue pair space to minimize time-wait impacts. Virtual queue pairs are allocated from a virtual queue pair pool of a node to connections between the node and other nodes. The connection is established between a physical queue pair of the node and physical queue pairs of other nodes. From the viewpoint of the other nodes, they are communicating with the present node using the virtual queue pair and not the physical queue pair for the present node. By using the virtual queue pairs, the same physical queue pair may accommodate multiple connections with other nodes simultaneously. Moreover, when a connection is torn down, the virtual queue pair is placed in a time-wait state rather than the physical queue pair. As a result, the physical queue pair may continue to function while the virtual queue pair is in the time-wait state.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention is directed to an improved data processing system.More specifically, the present invention is directed to an apparatus andmethod for virtualizing a queue pair space to minimize time-waitimpacts.

2. Description of Related Art

In a System Area Network (SAN), the hardware provides a message passingmechanism that can be used for Input/Output devices (I/O) andinterprocess communications (IPC) between general computing nodes.Processes executing on devices access SAN message passing hardware byposting send/receive messages to send/receive work queues on a SANchannel adapter (CA). These processes also are referred to as“consumers.”

The send/receive work queues (WQ) are assigned to a consumer as a queuepair (QP). The messages can be sent over five different transport types:Reliable Connected (RC), Reliable datagram (RD), Unreliable Connected(UC), Unreliable Datagram (UD), and Raw Datagram (RawD). Consumersretrieve the results of these messages from a completion queue (CQ)through SAN send and receive work completion (WC) queues. The sourcechannel adapter takes care of segmenting outbound messages and sendingthem to the destination. The destination channel adapter takes care ofreassembling inbound messages and placing them in the memory spacedesignated by the destination's consumer.

Two channel adapter types are present in nodes of the SAN fabric, a hostchannel adapter (HCA) and a target channel adapter (TCA). The hostchannel adapter is used by general purpose computing nodes to access theSAN fabric. Consumers use SAN verbs to access host channel adapterfunctions. The software that interprets verbs and directly accesses thechannel adapter is known as the channel interface (CI).

Target channel adapters (TCA) are used by nodes that are the subject ofmessages sent from host channel adapters. The target channel adaptersserve a similar function as that of the host channel adapters inproviding the target node an access point to the SAN fabric.

The SAN channel adapter architecture explicitly provides for sending andreceiving messages directly from application programs running under anoperating system. No intervention by the operating system is requiredfor an application program to post messages on send queues, post messagereceive buffers on receive queues, and detect completion of send orreceive operations by polling of completion queues or detecting theevent of an entry stored on a completion queue, e.g., via an interrupt.

When connections are established between nodes in a SAN fabric, physicalqueue pairs of channel adapters are typically used to facilitate theconnection. When these connections are torn down, the physical queuepairs are placed in a time-wait state in order to make sure that alldata packets in the SAN fabric at the time the connection is torn down,have time to be routed to their destination. During this time-waitstate, the physical queue pairs cannot be used to establish newconnections with the same or other nodes. This results in aninefficiency in the SAN architecture with regard to the establishmentand tearing down of connections between nodes. Therefore, it would bebeneficial to have an apparatus and method for avoiding the time-waitstate delays experienced in typical SAN architectures.

SUMMARY OF THE INVENTION

An apparatus and method for virtualizing a queue pair space to minimizetime-wait impacts. The apparatus and method allocate virtual queue pairsfrom a virtual queue pair pool of a node to connections between the nodeand other nodes.

The connection is established between a physical queue pair of the nodeand a physical queue pair of another node. However, from the viewpointof other nodes, they are communicating with the present node using thevirtual queue pair and not the physical queue pair for the present node.

By using the virtual queue pairs, the same physical queue pair may beused for successive connections without the need to wait for thetimewait period to elapse. Moreover, by using a virtual queue pairrather than a physical queue pair, when a connection is torn down, thevirtual queue pair is placed in a time-wait state rather than thephysical queue pair. As a result, the physical queue pair may continueto function while the virtual queue pair is in the time-wait state

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a diagram of a distributed computer system is illustrated inaccordance with a preferred embodiment of the present invention;

FIG. 2 is a functional block diagram of a host processor node inaccordance with a preferred embodiment of the present invention;

FIG. 3A is a diagram of a host channel adapter in accordance with apreferred embodiment of the present invention;

FIG. 3B is a diagram of a switch in accordance with a preferredembodiment of the present invention;

FIG. 3C is a diagram of a router in accordance with a preferredembodiment of the present invention;

FIG. 4 is a diagram illustrating processing of work requests inaccordance with a preferred embodiment of the present invention;

FIG. 5 is a diagram illustrating a portion of a distributed computersystem in accordance with a preferred embodiment of the presentinvention in which a reliable connection service is used;

FIG. 6 is a diagram illustrating a portion of a distributed computersystem in accordance with a preferred embodiment of the presentinvention in which reliable datagram service connections are used;

FIG. 7 is an illustration of a data packet in accordance with apreferred embodiment of the present invention;

FIG. 8 is a diagram illustrating a portion of a distributed computersystem in accordance with a preferred embodiment of the presentinvention;

FIG. 9 is a diagram illustrating the network addressing used in adistributed networking system in accordance with the present invention;

FIG. 10 is a diagram illustrating a portion of a distributed computingsystem in accordance with a preferred embodiment of the presentinvention in which the structure of SAN fabric subnets is illustrated;

FIG. 11 is a diagram of a layered communication architecture used in apreferred embodiment of the present invention;

FIG. 12 is an exemplary diagram illustrating a process for creating aconnection in a SAN architecture;

FIG. 13 is an exemplary diagram illustrating a process for releasing ortearing down a connection in a SAN architecture;

FIG. 14 is an exemplary diagram illustrating a connection between twonodes in accordance with the present invention;

FIG. 15 is an exemplary diagram illustrating the re-use of virtualizedqueue pairs before a time-wait period expires;

FIG. 16 is a flowchart outlining an exemplary operation of the presentinvention; and

FIG. 17 is an exemplary diagram illustrating the application of thepresent invention to end-to-end contexts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides an apparatus and method for virtualizinga queue pair space to minimize time-wait impacts. The present inventionmay be implemented in hardware, software, or a combination of hardwareand software. The present invention is preferably implemented in adistributed computing system, such as a system area network (SAN) havingend nodes, switches, routers, and links interconnecting thesecomponents. Each end node uses send and receive queue pairs to transmitand receives messages. The end nodes segment the message into packetsand transmit the packets over the links. The switches and routersinterconnect the end nodes and route the packets to the appropriate endnode. The end nodes reassemble the packets into a message at thedestination.

FIG. 1 is a diagram of a distributed computer system in accordance witha preferred embodiment of the present invention. The distributedcomputer system represented in FIG. 1 takes the form of a system areanetwork (SAN) 100 and is provided merely for illustrative purposes, andthe embodiments of the present invention described below can beimplemented on computer systems of numerous other types andconfigurations. For example, computer systems implementing the presentinvention can range from a small server with one processor and a fewinput/output (I/O) adapters to massively parallel supercomputer systemswith hundreds or thousands of processors and thousands of I/O adapters.Furthermore, the present invention can be implemented in aninfrastructure of remote computer systems connected by an Internet orintranet.

SAN 100 is a high-bandwidth, low-latency network interconnecting nodeswithin the distributed computer system. A node is any component attachedto one or more links of a network and forming the origin and/ordestination of messages within the network. In the depicted example, SAN100 includes nodes in the form of host processor node 102, hostprocessor node 104, redundant array independent disk (RAID) subsystemnode 106, and I/O chassis node 108. The nodes illustrated in FIG. 1 arefor illustrative purposes only, as SAN 100 can connect any number andany type of independent processor nodes, I/O adapter nodes, and I/Odevice nodes. Any one of the nodes can function as an endnode, which isherein defined to be a device that originates or finally consumesmessages or frames in SAN 100.

In one embodiment of the present invention, an error handling mechanismin distributed computer systems is present in which the error handlingmechanism allows for reliable connection or reliable datagramcommunication between end nodes in distributed computing system, such asSAN 100.

A message, as used herein, is an application-defined unit of dataexchange, which is a primitive unit of communication between cooperatingprocesses. A packet is one unit of data encapsulated by networkingprotocol headers and/or trailers. The headers generally provide controland routing information for directing the frame through SAN. The trailergenerally contains control and cyclic redundancy check (CRC) data forensuring packets are not delivered with corrupted contents.

SAN 100 contains the communications and management infrastructuresupporting both I/O and interprocessor communications (IPC) within adistributed computer system. The SAN 100 shown in FIG. 1 includes aswitched communications fabric 116, which allows many devices toconcurrently transfer data with high-bandwidth and low latency in asecure, remotely managed environment. Endnodes can communicate overmultiple ports and utilize multiple paths through the SAN fabric. Themultiple ports and paths through the SAN shown in FIG. 1 can be employedfor fault tolerance and increased bandwidth data transfers.

The SAN 100 in FIG. 1 includes switch 112, switch 114, switch 146, androuter 117. A switch is a device that connects multiple links togetherand allows routing of packets from one link to another link within asubnet using a small header Destination Local Identifier (DLID) field. Arouter is a device that connects multiple subnets together and iscapable of routing frames from one link in a first subnet to anotherlink in a second subnet using a large header Destination Globally UniqueIdentifier (DGUID).

In one embodiment, a link is a full duplex channel between any twonetwork fabric elements, such as endnodes, switches, or routers. Examplesuitable links include, but are not limited to, copper cables, opticalcables, and printed circuit copper traces on backplanes and printedcircuit boards.

For reliable service types, endnodes, such as host processor endnodesand I/O adapter endnodes, generate request packets and returnacknowledgment packets. Switches and routers pass packets along, fromthe source to the destination. Except for the variant CRC trailer field,which is updated at each stage in the network, switches pass the packetsalong unmodified. Routers update the variant CRC trailer field andmodify other fields in the header as the packet is routed.

In SAN 100 as illustrated in FIG. 1, host processor node 102, hostprocessor node 104, and I/O chassis 108 include at least one channeladapter (CA) to interface to SAN 100. In one embodiment, each channeladapter is an endpoint that implements the channel adapter interface insufficient detail to source or sink packets transmitted on SAN fabric100. Host processor node 102 contains channel adapters in the form ofhost channel adapter 118 and host channel adapter 120. Host processornode 104 contains host channel adapter 122 and host channel adapter 124.Host processor node 102 also includes central processing units 126–130and a memory 132 interconnected by bus system 134. Host processor node104 similarly includes central processing units 136–140 and a memory 142interconnected by a bus system 144.

Host channel adapters 118 and 120 provide a connection to switch 112while host channel adapters 122 and 124 provide a connection to switches112 and 114.

In one embodiment, a host channel adapter is implemented in hardware. Inthis implementation, the host channel adapter hardware offloads much ofcentral processing unit and I/O adapter communication overhead. Thishardware implementation of the host channel adapter also permitsmultiple concurrent communications over a switched network without thetraditional overhead associated with communicating protocols. In oneembodiment, the host channel adapters and SAN 100 in FIG. 1 provide theI/O and interprocessor communications (IPC) consumers of the distributedcomputer system with zero processor-copy data transfers withoutinvolving the operating system kernel process, and employs hardware toprovide reliable, fault tolerant communications.

As indicated in FIG. 1, router 117 is coupled to wide area network (WAN)and/or local area network (LAN) connections to other hosts or otherrouters. The I/O chassis 108 in FIG. 1 includes an I/O switch 146 andmultiple I/O modules 148–156. In these examples, the I10 modules takethe form of adapter cards. Example adapter cards illustrated in FIG. 1include a SCSI adapter card for I/O module 148; an adapter card to fiberchannel hub and fiber channel-arbitrated loop (FC-AL) devices for I/Omodule 152; an ethernet adapter card for I/O module 150; a graphicsadapter card for I/O module 154; and a video adapter card for I/O module156. Any known type of adapter card can be implemented. I/O adaptersalso include a switch in the I/O adapter backplane to couple the adaptercards to the SAN fabric. These modules contain target channel adapters158–166.

In this example, RAID subsystem node 106 in FIG. 1 includes a processor168, a memory 170, a target channel adapter (TCA) 172, and multipleredundant and/or striped storage disk unit 174. Target channel adapter172 can be a fully functional host channel adapter.

SAN 100 handles data communications for I/O and interprocessorcommunications. SAN 100 supports high-bandwidth and scalability requiredfor I/O and also supports the extremely low latency and low CPU overheadrequired for interprocessor communications. User clients can bypass theoperating system kernel process and directly access networkcommunication hardware, such as host channel adapters, which enableefficient message passing protocols. SAN 100 is suited to currentcomputing models and is a building block for new forms of I/O andcomputer cluster communication. Further, SAN 100 in FIG. 1 allows I/Oadapter nodes to communicate among themselves or communicate with any orall of the processor nodes in distributed computer system. With an I/Oadapter attached to the SAN 100, the resulting I/O adapter node hassubstantially the same communication capability as any host processornode in SAN 100.

In one embodiment, the SAN 100 shown in FIG. 1 supports channelsemantics and memory semantics. Channel semantics is sometimes referredto as send/receive or push communication operations. Channel semanticsare the type of communications employed in a traditional I/O channelwhere a source device pushes data and a destination device determines afinal destination of the data. In channel semantics, the packettransmitted from a source process specifies a destination processes'communication port, but does not specify where in the destinationprocesses' memory space the packet will be written. Thus, in channelsemantics, the destination process pre-allocates where to place thetransmitted data.

In memory semantics, a source process directly reads or writes thevirtual address space of a remote node destination process. The remotedestination process need only communicate the location of a buffer fordata, and does not need to be involved in the transfer of any data.Thus, in memory semantics, a source process sends a data packetcontaining the destination buffer memory address of the destinationprocess. In memory semantics, the destination process previously grantspermission for the source process to access its memory.

Channel semantics and memory semantics are typically both necessary forI/O and interprocessor communications. A typical I/O operation employs acombination of channel and memory semantics. In an illustrative exampleI/O operation of the distributed computer system shown in FIG. 1, a hostprocessor node, such as host processor node 102, initiates an I/Ooperation by using channel semantics to send a disk write command to adisk I/O adapter, such as RAID subsystem target channel adapter (TCA)172. The disk I/O adapter examines the command and uses memory semanticsto read the data buffer directly from the memory space of the hostprocessor node. After the data buffer is read, the disk I/O adapteremploys channel semantics to push an I/O completion message back to thehost processor node.

In one exemplary embodiment, the distributed computer system shown inFIG. 1 performs operations that employ virtual addresses and virtualmemory protection mechanisms to ensure correct and proper access to allmemory. Applications running in such a distributed computed system arenot required to use physical addressing for any operations.

Turning next to FIG. 2, a functional block diagram of a host processornode is depicted in accordance with a preferred embodiment of thepresent invention. Host processor node 200 is an example of a hostprocessor node, such as host processor node 102 in FIG. 1. In thisexample, host processor node 200 shown in FIG. 2 includes a set ofconsumers 202–208, which are processes executing on host processor node200. Host processor node 200 also includes channel adapter 210 andchannel adapter 212. Channel adapter 210 contains ports 214 and 216while channel adapter 212 contains ports 218 and 220. Each port connectsto a link. The ports can connect to one SAN subnet or multiple SANsubnets, such as SAN 100 in FIG. 1. In these examples, the channeladapters take the form of host channel adapters.

Consumers 202–208 transfer messages to the SAN via the verbs interface222 and message and data service 224. A verbs interface is essentiallyan abstract description of the functionality of a host channel adapter.An operating system may expose some or all of the verb functionalitythrough its programming interface. Basically, this interface defines thebehavior of the host. Additionally, host processor node 200 includes amessage and data service 224, which is a higher-level interface than theverb layer and is used to process messages and data received throughchannel adapter 210 and channel adapter 212. Message and data service224 provides an interface to consumers 202–208 to process messages andother data.

With reference now to FIG. 3A, a diagram of a host channel adapter isdepicted in accordance with a preferred embodiment of the presentinvention. Host channel adapter 300A shown in FIG. 3A includes a set ofqueue pairs (QPs) 302A–310A, which are used to transfer messages to thehost channel adapter ports 312A–316A. Buffering of data to host channeladapter ports 312A14 316A is channeled through virtual lanes (VL)318A–334A where each VL has its own flow control. Subnet managerconfigures channel adapters with the local addresses for each physicalport, i.e., the port's LID. Subnet manager agent (SMA) 336A is theentity that communicates with the subnet manager for the purpose ofconfiguring the channel adapter. Memory translation and protection (MTP)338A is a mechanism that translates virtual addresses to physicaladdresses and validates access rights. Direct memory access (DMA) 340Aprovides for direct memory access operations using memory 340A withrespect to queue pairs 302A–310A.

A single channel adapter, such as the host channel adapter 300A shown inFIG. 3A, can support thousands of queue pairs. By contrast, a targetchannel adapter in an I/O adapter typically supports a much smallernumber of queue pairs. Each queue pair consists of a send work queue(SWQ) and a receive work queue. The send work queue is used to sendchannel and memory semantic messages. The receive work queue receiveschannel semantic messages. A consumer calls an operating-system specificprogramming interface, which is herein referred to as verbs, to placework requests (WRs) onto a work queue.

FIG. 3B depicts a switch 300B in accordance with a preferred embodimentof the present invention. Switch 300B includes a packet relay 302B incommunication with a number of ports 304B through virtual lanes such asvirtual lane 306B. Generally, a switch such as switch 300B can routepackets from one port to any other port on the same switch.

Similarly, FIG. 3C depicts a router 300C according to a preferredembodiment of the present invention. Router 300C includes a packet relay302C in communication with a number of ports 304C through virtual lanessuch as virtual lane 306C. Like switch 300B, router 300C will generallybe able to route packets from one port to any other port on the samerouter.

Channel adapters, switches, and routers employ multiple virtual laneswithin a single physical link. As illustrated in FIGS. 3A, 3B, and 3C,physical ports connect endnodes, switches, and routers to a subnet.Packets injected into the SAN fabric follow one or more virtual lanesfrom the packet's source to the packet's destination. The virtual lanethat is selected is mapped from a service level associated with thepacket. At any one time, only one virtual lane makes progress on a givenphysical link. Virtual lanes provide a technique for applying link levelflow control to one virtual lane without affecting the other virtuallanes. When a packet on one virtual lane blocks due to contention,quality of service (QoS), or other considerations, a packet on adifferent virtual lane is allowed to make progress. Virtual lanes areemployed for numerous reasons, some of which are as follows: Virtuallanes provide QoS. In one example embodiment, certain virtual lanes arereserved for high priority or isochronous traffic to provide QoS.

Virtual lanes provide deadlock avoidance. Virtual lanes allow topologiesthat contain loops to send packets across all physical links and stillbe assured the loops won't cause back pressure dependencies that mightresult in deadlock.

Virtual lanes alleviate head-of-line blocking. When a switch has no morecredits available for packets that utilize a given virtual lane, packetsutilizing a different virtual lane that has sufficient credits areallowed to make forward progress.

With reference now to FIG. 4, a diagram illustrating processing of workrequests is depicted in accordance with a preferred embodiment of thepresent invention. In FIG. 4, a receive work queue 400, send work queue402, and completion queue 404 are present for processing requests fromand for consumer 406. These requests from consumer 402 are eventuallysent to hardware 408. In this example, consumer 406 generates workrequests 410 and 412 and receives work completion 414. As shown in FIG.4, work requests placed onto a work queue are referred to as work queueelements (WQEs).

Send work queue 402 contains work queue elements (WQEs) 422–428,describing data to be transmitted on the SAN fabric. Receive work queue400 contains work queue elements (WQEs) 416–420, describing where toplace incoming channel semantic data from the SAN fabric. A work queueelement is processed by hardware 408 in the host channel adapter.

The verbs also provide a mechanism for retrieving completed work fromcompletion queue 404. As shown in FIG. 4, completion queue 404 containscompletion queue elements (CQEs) 430–436. Completion queue elementscontain information about previously completed work queue elements.Completion queue 404 is used to create a single point of completionnotification for multiple queue pairs. A completion queue element is adata structure on a completion queue. This element describes a completedwork queue element. The completion queue element contains sufficientinformation to determine the queue pair and specific work queue elementthat completed. A completion queue context is a block of informationthat contains pointers to, length, and other information needed tomanage the individual completion queues.

Example work requests supported for the send work queue 402 shown inFIG. 4 are as follows. A send work request is a channel semanticoperation to push a set of local data segments to the data segmentsreferenced by a remote node's receive work queue element. For example,work queue element 428 contains references to data segment 4 438, datasegment 5 440, and data segment 6 442. Each of the send work request'sdata segments contains a virtually contiguous memory region. The virtualaddresses used to reference the local data segments are in the addresscontext of the process that created the local queue pair.

A remote direct memory access (RDMA) read work request provides a memorysemantic operation to read a virtually contiguous memory space on aremote node. A memory space can either be a portion of a memory regionor portion of a memory window. A memory region references a previouslyregistered set of virtually contiguous memory addresses defined by avirtual address and length. A memory window references a set ofvirtually contiguous memory addresses that have been bound to apreviously registered region.

The RDMA Read work request reads a virtually contiguous memory space ona remote endnode and writes the data to a virtually contiguous localmemory space. Similar to the send work request, virtual addresses usedby the RDMA Read work queue element to reference the local data segmentsare in the address context of the process that created the local queuepair. For example, work queue element 416 in receive work queue 400references data segment 1 444, data segment 2 446, and data segment 448.The remote virtual addresses are in the address context of the processowning the remote queue pair targeted by the RDMA Read work queueelement.

A RDMA Write work queue element provides a memory semantic operation towrite a virtually contiguous memory space on a remote node. The RDMAWrite work queue element contains a scatter list of local virtuallycontiguous memory spaces and the virtual address of the remote memoryspace into which the local memory spaces are written.

A RDMA FetchOp work queue element provides a memory semantic operationto perform an atomic operation on a remote word. The RDMA FetchOp workqueue element is a combined RDMA Read, Modify, and RDMA Write operation.The RDMA FetchOp work queue element can support severalread-modify-write operations, such as Compare and Swap if equal.

A bind (unbind) remote access key (R_Key) work queue element provides acommand to the host channel adapter hardware to modify (destroy) amemory window by associating (disassociating) the memory window to amemory region. The R_Key is part of each RDMA access and is used tovalidate that the remote process has permitted access to the buffer.

In one embodiment, receive work queue 400 shown in FIG. 4 only supportsone type of work queue element, which is referred to as a receive workqueue element. The receive work queue element provides a channelsemantic operation describing a local memory space into which incomingsend messages are written. The receive work queue element includes ascatter list describing several virtually contiguous memory spaces. Anincoming send message is written to these memory spaces. The virtualaddresses are in the address context of the process that created thelocal queue pair.

For interprocessor communications, a user-mode software processtransfers data through queue pairs directly from where the bufferresides in memory. In one embodiment, the transfer through the queuepairs bypasses the operating system and consumes few host instructioncycles. Queue pairs permit zero processor-copy data transfer with nooperating system kernel involvement. The zero processor-copy datatransfer provides for efficient support of high-bandwidth andlow-latency communication.

When a queue pair is created, the queue pair is set to provide aselected type of transport service. In one embodiment, a distributedcomputer system implementing the present invention supports four typesof transport services: reliable, unreliable, reliable datagram, andunreliable datagram connection service.

Reliable and Unreliable connected services associate a local queue pairwith one and only one remote queue pair. Connected services require aprocess to create a queue pair for each process that is to communicatewith over the SAN fabric. Thus, if each of N host processor nodescontain P processes, and all P processes on each node wish tocommunicate with all the processes on all the other nodes, each hostprocessor node requires P²×(N−1) queue pairs. Moreover, a process canconnect a queue pair to another queue pair on the same host channeladapter.

A portion of a distributed computer system employing a reliableconnection service to communicate between distributed processes isillustrated generally in FIG. 5. The distributed computer system 500 inFIG. 5 includes a host processor node 1, a host processor node 2, and ahost processor node 3. Host processor node 1 includes a process A 510.Host processor node 3 includes a process C 520 and a process D 530. Hostprocessor node 2 includes a process E 540.

Host processor node 1 includes queue pairs 4, 6 and 7, each having asend work queue and receive work queue. Host processor node 2 has aqueue pair 9 and host processor node 3 has queue pairs 2 and 5. Thereliable connection service of distributed computer system 500associates a local queue pair with one and only one remote queue pair.Thus, the queue pair 4 is used to communicate with queue pair 2; queuepair 7 is used to communicate with queue pair 5; and queue pair 6 isused to communicate with queue pair 9.

A WQE placed on one queue pair in a reliable connection service causesdata to be written into the receive memory space referenced by a ReceiveWQE of the connected queue pair. RDMA operations operate on the addressspace of the connected queue pair.

In one embodiment of the present invention, the reliable connectionservice is made reliable because hardware maintains sequence numbers andacknowledges all packet transfers. A combination of hardware and SANdriver software retries any failed communications. The process client ofthe queue pair obtains reliable communications even in the presence ofbit errors, receive underruns, and network congestion. If alternativepaths exist in the SAN fabric, reliable communications can be maintainedeven in the presence of failures of fabric switches, links, or channeladapter ports.

In addition, acknowledgments may be employed to deliver data reliablyacross the SAN fabric. The acknowledgment may, or may not, be a processlevel acknowledgment, i.e. an acknowledgment that validates that areceiving process has consumed the data. Alternatively, theacknowledgment may be one that only indicates that the data has reachedits destination.

Reliable datagram service associates a local end-to-end context (EEC)with one and only one remote end-to-end context. The reliable datagramservice permits a client process of one queue pair to communicate withany other queue pair on any other remote node. At a receive work queue,the reliable datagram service permits incoming messages from any sendwork queue on any other remote node.

The reliable datagram service greatly improves scalability because thereliable datagram service is connectionless. Therefore, an endnode witha fixed number of queue pairs can communicate with far more processesand endnodes with a reliable datagram service than with a reliableconnection transport service. For example, if each of N host processornodes contain P processes, and all P processes on each node wish tocommunicate with all the processes on all the other nodes, the reliableconnection service requires P²×(N−1) queue pairs on each node. Bycomparison, the connectionless reliable datagram service only requires Pqueue pairs+(N−1) EE contexts on each node for exactly the samecommunications.

A portion of a distributed computer system employing a reliable datagramservice to communicate between distributed processes is illustrated inFIG. 6. The distributed computer system 600 in FIG. 6 includes a hostprocessor node 1, a host processor node 2, and a host processor node 3.Host processor node 1 includes a process A 610 having a queue pair 4.Host processor node 2 has a process C 620 having a queue pair 24 and aprocess D 630 having a queue pair 25. Host processor node 3 has aprocess E 640 having a queue pair 14.

In the reliable datagram service implemented in the distributed computersystem 600, the queue pairs are coupled in what is referred to as aconnectionless transport service. For example, a reliable datagramservice couples queue pair 4 to queue pairs 24, 25 and 14. Specifically,a reliable datagram service allows queue pair 4's send work queue toreliably transfer messages to receive work queues in queue pairs 24, 25and 14. Similarly, the send queues of queue pairs 24, 25, and 14 canreliably transfer messages to the receive work queue in queue pair 4.

In one embodiment of the present invention, the reliable datagramservice employs sequence numbers and acknowledgments associated witheach message frame to ensure the same degree of reliability as thereliable connection service. End-to-end (EE) contexts maintainend-to-end specific state to keep track of sequence numbers,acknowledgments, and time-out values. The end-to-end state held in theEE contexts is shared by all the connectionless queue pairscommunication between a pair of endnodes. Each endnode requires at leastone EE context for every endnode it wishes to communicate with in thereliable datagram service (e.g., a given endnode requires at least N EEcontexts to be able to have reliable datagram service with N otherendnodes).

The unreliable datagram service is connectionless. The unreliabledatagram service is employed by management applications to discover andintegrate new switches, routers, and endnodes into a given distributedcomputer system. The unreliable datagram service does not provide thereliability guarantees of the reliable connection service and thereliable datagram service. The unreliable datagram service accordinglyoperates with less state information maintained at each endnode.

Turning next to FIG. 7, an illustration of a data packet is depicted inaccordance with a preferred embodiment of the present invention. A datapacket is a unit of information that is routed through the SAN fabric.The data packet is an endnode-to-endnode construct, and is thus createdand consumed by endnodes. For packets destined to a channel adapter(either host or target), the data packets are neither generated norconsumed by the switches and routers in the SAN fabric. Instead for datapackets that are destined to a channel adapter, switches and routerssimply move request packets or acknowledgment packets closer to theultimate destination, modifying the variant link header fields in theprocess. Routers, also modify the packet's network header when thepacket crosses a subnet boundary. In traversing a subnet, a singlepacket stays on a single service level.

Message data 700 contains data segment 1 702, data segment 2 704, anddata segment 3 706, which are similar to the data segments illustratedin FIG. 4. In this example, these data segments form a packet 708, whichis placed into packet payload 710 within data packet 712. Additionally,data packet 712 contains CRC 714, which is used for error checking.Additionally, routing header 716 and transport 718 are present in datapacket 712. Routing header 716 is used to identify source anddestination ports for data packet 712. Transport header 718 in thisexample specifies the destination queue pair for data packet 712.Additionally, transport header 718 also provides information such as theoperation code, packet sequence number, and partition for data packet712.

The operating code identifies whether the packet is the first, last,intermediate, or only packet of a message. The operation code alsospecifies whether the operation is a send RDMA write, read, or atomic.The packet sequence number is initialized when communication isestablished and increments each time a queue pair creates a new packet.Ports of an endnode may be configured to be members of one or morepossibly overlapping sets called partitions.

In FIG. 8, a portion of a distributed computer system is depicted at 800to illustrate an example request and acknowledgment transaction. Thedistributed computer system in FIG. 8 includes a host processor node 802and a host processor node 804. Host processor node 802 includes a hostchannel adapter 806. Host processor node 804 includes a host channeladapter 808. The distributed computer system in FIG. 8 includes a SANfabric 810, which includes a switch 812 and a switch 814. The SAN fabricincludes a link coupling host channel adapter 806 to switch 812; a linkcoupling switch 812 to switch 814; and a link coupling host channeladapter 808 to switch 814.

In the example transactions, host processor node 802 includes a clientprocess A 816. Host processor node 804 includes a client process B 818.Client process A interacts with host channel adapter hardware 806through queue pair 824 and 826. Client process B interacts with hardwarechannel adapter hardware 808 through queue pair 828 and 830. Queue pairs824 and 828 are data structures that include a send work queue and areceive work queue. Process A initiates a message request by postingwork queue elements to the send queue of queue pair 824. Such a workqueue element is illustrated in FIG. 4. The message request of clientprocess A is referenced by a gather list contained in the send workqueue element. Each data segment in the gather list points to avirtually contiguous local memory region, which contains a part of themessage, such as indicated by data segments 1, 2, and 3, whichrespectively hold message parts 1, 2, and 3, in FIG. 4.

Hardware in host channel adapter 806 reads the work queue element andsegments the message stored in virtual contiguous buffers into datapackets, such as the data packet illustrated in FIG. 7. Data packets arerouted through the SAN fabric, and for reliable transfer services, areacknowledged by the final destination endnode. If not successivelyacknowledged, the data packet is retransmitted by the source endnode.Data packets are generated by source endnodes and consumed bydestination endnodes.

In reference to FIG. 9, a diagram illustrating the network addressingused in a distributed networking system is depicted in accordance withthe present invention. A host name provides a logical identification fora host node, such as a host processor node or I/O adapter node. The hostname identifies the endpoint for messages such that messages aredestined for processes residing on an end node specified by the hostname. Thus, there is one host name per node, but a node can havemultiple CAs. A single IEEE assigned 64-bit identifier (EUI-64) 902 isassigned to each component. A component can be a switch, router, or CA.

One or more globally unique ID (GUID) identifiers 904 are assigned perCA port 906. Multiple GUIDs (a.k.a. IP addresses) can be used forseveral reasons, some of which are illustrated by the followingexamples. In one embodiment, different IP addresses identify differentpartitions or services on an end node. In a different embodiment,different IP addresses are used to specify different Quality of Service(QoS) attributes. In yet another embodiment, different IP addressesidentify different paths through intra-subnet routes.

One GUID 908 is assigned to a switch 910.

A local ID (LID) refers to a short address ID used to identify a CA portwithin a single subnet. In one example embodiment, a subnet has up to2¹⁶ end nodes, switches, and routers, and the LID is accordingly 16bits. A source LID (SLID) and a destination LID (DUD) are the source anddestination LIDs used in a local network header. A single CA port 906has up to 2^(LMC) LIDs 912 assigned to it. The LMC represents the LIDMask Control field in the CA. A mask is a pattern of bits used to acceptor reject bit patterns in another set of data.

Multiple LIDs can be used for several reasons some of which are providedby the following examples. In one embodiment, different LIDs identifydifferent partitions or services in an end node. In another embodiment,different LIDs are used to specify different QoS attributes. In yet afurther embodiment, different LIDs specify different paths through thesubnet. A single switch port 914 has one LID 916 associated with it.

A one-to-one correspondence does not necessarily exist between LIDs andGUIDs, because a CA can have more or less LIDs than GUIDs for each port.For CAs with redundant ports and redundant connectivity to multiple SANfabrics, the CAs can, but are not required to, use the same LID and GUIDon each of its ports.

A portion of a distributed computer system in accordance with apreferred embodiment of the present invention is illustrated in FIG. 10.Distributed computer system 1000 includes a subnet 1002 and a subnet1004. Subnet 1002 includes host processor nodes 1006, 1008, and 1010.Subnet 1004 includes host processor nodes 1012 and 1014. Subnet 1002includes switches 1016 and 1018. Subnet 1004 includes switches 1020 and1022.

Routers connect subnets. For example, subnet 1002 is connected to subnet1004 with routers 1024 and 1026. In one example embodiment, a subnet hasup to 216 endnodes, switches, and routers.

A subnet is defined as a group of endnodes and cascaded switches that ismanaged as a single unit. Typically, a subnet occupies a singlegeographic or functional area. For example, a single computer system inone room could be defined as a subnet. In one embodiment, the switchesin a subnet can perform very fast wormhole or cut-through routing formessages.

A switch within a subnet examines the DLID that is unique within thesubnet to permit the switch to quickly and efficiently route incomingmessage packets. In one embodiment, the switch is a relatively simplecircuit, and is typically implemented as a single integrated circuit. Asubnet can have hundreds to thousands of endnodes formed by cascadedswitches.

As illustrated in FIG. 10, for expansion to much larger systems, subnetsare connected with routers, such as routers 1024 and 1026. The routerinterprets the IP destination ID (e.g., IPv6 destination ID) and routesthe IP-like packet.

An example embodiment of a switch is illustrated generally in FIG. 3B.Each I/O path on a switch or router has a port. Generally, a switch canroute packets from one port to any other port on the same switch.

Within a subnet, such as subnet 1002 or subnet 1004, a path from asource port to a destination port is determined by the LID of thedestination host channel adapter port. Between subnets, a path isdetermined by the IP address (e.g., IPv6 address) of the destinationhost channel adapter port and by the LID address of the router portwhich will be used to reach the destination's subnet.

In one embodiment, the paths used by the request packet and the requestpacket's corresponding positive acknowledgment (ACK) or negativeacknowledgment (NAK) frame are not required to be symmetric. In oneembodiment employing certain routing, switches select an output portbased on the DLID. In one embodiment, a switch uses one set of routingdecision criteria for all its input ports. In one example embodiment,the routing decision criteria are contained in one routing table. In analternative embodiment, a switch employs a separate set of criteria foreach input port.

A data transaction in the distributed computer system of the presentinvention is typically composed of several hardware and software steps.A client process data transport service can be a user-mode or akernel-mode process. The client process accesses host channel adapterhardware through one or more queue pairs, such as the queue pairsillustrated in FIGS. 3A, 5, and 6. The client process calls anoperating-system specific programming interface, which is hereinreferred to as “verbs.” The software code implementing verbs posts awork queue element to the given queue pair work queue.

There are many possible methods of posting a work queue element andthere are many possible work queue element formats, which allow forvarious cost/performance design points, but which do not affectinteroperability. A user process, however, must communicate to verbs ina well-defined manner, and the format and protocols of data transmittedacross the SAN fabric must be sufficiently specified to allow devices tointeroperate in a heterogeneous vendor environment.

In one embodiment, channel adapter hardware detects work queue elementpostings and accesses the work queue element. In this embodiment, thechannel adapter hardware translates and validates the work queueelement's virtual addresses and accesses the data.

An outgoing message is split into one or more data packets. In oneembodiment, the channel adapter hardware adds a transport header and anetwork header to each packet. The transport header includes sequencenumbers and other transport information. The network header includesrouting information, such as the destination IP address and othernetwork routing information. The link header contains the DestinationLocal Identifier (DLID) or other local routing information. Theappropriate link header is always added to the packet. The appropriateglobal network header is added to a given packet if the destinationendnode resides on a remote subnet.

If a reliable transport service is employed, when a request data packetreaches its destination endnode, acknowledgment data packets are used bythe destination endnode to let the request data packet sender know therequest data packet was validated and accepted at the destination.Acknowledgment data packets acknowledge one or more valid and acceptedrequest data packets. The requester can have multiple outstandingrequest data packets before it receives any acknowledgments. In oneembodiment, the number of multiple outstanding messages, i.e. Requestdata packets, is determined when a queue pair is created.

One embodiment of a layered architecture 1100 for implementing thepresent invention is generally illustrated in diagram form in FIG. 11.The layered architecture diagram of FIG. 11 shows the various layers ofdata communication paths, and organization of data and controlinformation passed between layers.

Host channel adapter endnode protocol layers (employed by endnode 1111,for instance) include an upper level protocol 1102 defined by consumer1103, a transport layer 1104; a network layer 1106, a link layer 1108,and a physical layer 1110. Switch layers (employed by switch 1113, forinstance) include link layer 1108 and physical layer 1110. Router layers(employed by router 1115, for instance) include network layer 1106, linklayer 1108, and physical layer 1110.

Layered architecture 1100 generally follows an outline of a classicalcommunication stack. With respect to the protocol layers of end node1111, for example, upper layer protocol 1102 employs verbs (1112) tocreate messages at transport layer 1104. Transport layer 1104 passesmessages (1114) to network layer 1106. Network layer 1106 routes packetsbetween network subnets (1116). Link layer 1108 routes packets within anetwork subnet (1118). Physical layer 1110 sends bits or groups of bitsto the physical layers of other devices. Each of the layers is unawareof how the upper or lower layers perform their functionality.

Consumers 1103 and 1105 represent applications or processes that employthe other layers for communicating between endnodes. Transport layer1104 provides end-to-end message movement. In one embodiment, thetransport layer provides four types of transport services as describedabove which are reliable connection service; reliable datagram service;unreliable datagram service; and raw datagram service. Network layer1106 performs packet routing through a subnet or multiple subnets todestination endnodes. Link layer 1108 performs flow-controlled, errorchecked, and prioritized packet delivery across links.

Physical layer 1110 performs technology-dependent bit transmission. Bitsor groups of bits are passed between physical layers via links 1122,1124, and 1126. Links can be implemented with printed circuit coppertraces, copper cable, optical cable, or with other suitable links.

The present invention operates within the SAN environment describedabove with regard to FIGS. 1–11. The present invention provides amechanism for virtualizing the queue pair space so that the impact ofTime-Wait is minimized.

FIG. 12 illustrates the process employed by a communication manager toestablish a connection between queue pairs (QPs) or end-to-end contexts(EECs) on two different nodes. Typically, the communication managerresides in the host processor node initiating the connection. However,the communication manager may reside in any other node and the presentinvention is not limited to any particular location of the communicationmanager.

As shown in FIG. 12, the communication management request (REQ) messageis used to request the establishment of the connection. This message issent on a well-known QP (such as QP1) that is monitored by managementagents on all nodes, i.e. applications on the nodes used to monitorqueue pairs. If the management agent residing on the receiving nodewishes to accept the request for the connection establishment, themanagement agent indicates this by responding with a communicationmanagement reply (REP) message. The management agent decides whether toaccept a request or not based on a number of considerations such aswhether it supports the service type requested in the REQ packet,whether it supports the transport service type requested, and whether ithas resources, such as a QP, available for the connection.

When the requesting communication manager receives the REP message, thecommunication manager indicates its receipt and that communication maybegin, by sending a ready-to-use (RTU) message. The REQ and REP messagescontain information that identifies the type of connection requestedand, in particular, the QP number (and EEC number for reliable datagramservice) that the sending node wishes to use for this connection.

FIG. 13 illustrates the process employed by a communication manager torelease a connection between QPs or EECs on two different nodes. Thecommunication management disconnect request (DREQ) message is used torequest the release of the connection. When the management agentresiding on the receiving node receives the DREQ, the management agentindicates that the connection may be released by responding with acommunication management disconnect reply (DREP) message. The DREQmessage contains an identifier that uniquely identifies the connectionto be released, and the QP number or EEC number of the remote node thatis associated with this connection.

When the responding node receives the DREQ, the responding node placesthe QP or EEC associated with this connection into a time-wait state.During the time-wait state, the QP or EEC may not be used for anotherconnection, so essentially it remains idle. The time-wait state is usedto ensure that all data packets being transmitted along the originalconnection are routed to their respective end nodes.

When the initiating node receives the DREP, the initiating node placesits QP or EEC associated with this connection into the time-wait state.The QP or EEC remains in the time-wait state for a sufficient time toallow any packets or acknowledgments to traverse the SAN fabric, so thatafter the time-wait period has elapsed, there will be no more packetsreceived for this connection. If the QP was reused immediately, a packetmay be received on the QP from the old connection (this is quitepossible as there may be packets in flight that were sent before theDREQ was received). This packet may be misinterpreted as being relatedto the new connection and may cause data integrity or security exposuresto the application.

FIG. 14 illustrates two nodes that have a connection established betweentwo QPs. Node 1 1410 that is using the virtualization process of thepresent invention, has a large pool of virtualized QPs 1450 from whichone of the virtualized QPs 1452 may be allocated to the connectionbetween node 1 1410 and node 2 1420. The pool of virtualized QPs 1450 isinitially allocated at initialization time when the HCA and it physicalQPs are initialized. There is a bit associated with each virtualized QPthat indicates whether it is available or in the time-wait state. Thisbit and the QP number itself are the only resources associated with thevirtualized QP. This pool 1450 may be large and may have any number ofvirtual QPs. For example, the pool 1450 may use up to the full 24 bitnumber space for queue pairs, as it does not consume any real resourcesin the channel adapter (CA). The physical QP space is much smaller as itrequires hardware resources in the CA that are used for the transmissionand reception of packets.

When a virtualized QP, such as virtual QP 1452, is allocated to aconnection, that QP number is associated with the physical QP 1460 thatis used for the connection. The particular QP allocated to a connectionis selected using a selection scheme. The selection scheme may be anyknown selection scheme, such as simply allocating the next QP in thestack that is not being used and is not in a time-wait state, using arandom selection scheme, or the like. The communication managerrequesting the connection typically performs the allocation of the QP.

In the depicted example, Node 2 1420 is not using the virtualizationprocess of the present invention and thus, only associates a physical QPnumber with the connection. While FIG. 14 shows only Node 1 1410 usingthe virtualized QPs of the present invention, the invention is notlimited to such an exemplary embodiment. Rather, both Nodes 1 and 2 maymake use of the present invention. In addition, there may be any numberof nodes upon which the present invention may be implemented and thepresent invention is not limited to only connections between two nodes.

As part of the virtualization process, Node 1 1410 allocates QP2 1452from the larger pool of virtualized QPs 1450 and associates the virtualqueue pair QP2 1452 with the physical queue pair QP 1460. The differencebetween the virtual queue pair QP2 1452 and the physical queue pair QP1460 is that the physical QP is where the WQEs are stored and thevirtualized QP only consumes a QP number and a bit to indicate whetherthe virtual QP is available.

In all communication management messages associated with this connectionthe communication manager, which may be in either one of node 1 or node2, identifies Node 1's QP as virtual queue pair QP 2 1452, not physicalqueue pair QP 1460. As depicted in FIG. 14, Node 1 1410 has virtualqueue pair QP 2 1452 connected to physical queue pair QP 4 1470 on Node2 1410. From Node 2's perspective, it has physical queue pair QP4 1470connected to virtual queue pair QP2 1452 on Node 1, and knows nothingabout the existence of physical queue pair QP 1460.

When the connection between virtual queue pair QP2 1452 and physicalqueue pair QP4 1470 is no longer needed, the communication managerrequests that the connection be disconnected using the DREQ and DREPprotocol described earlier. The virtual queue pair QP2 1452 and physicalqueue pair QP4 1470 are both placed in the time-wait state, and cannotbe used again until the time-wait period has expired.

Now, suppose, as illustrated in FIG. 15, Process A on Node 1 1510 needsto establish a connection with Process D on Node 2 1520. Node 2 1520cannot re-use the physical queue pair QP4 1570 for this connection untilthe time-wait period has expired. Therefore, Node 2 1520 must useanother physical queue pair, if there is one available.

As shown in FIG. 15, the physical queue pair QP5 1580 is allocated tothe new connection. Similarly, Node 1 1510 cannot re-use virtual queuepair QP2 1552 until the time-wait period has expired. However, QP2 1552is a virtualized QP and is not consuming CA hardware resources. Thus,another virtual queue pair QP from the virtualized pool 1550 may beassigned to the new connection, e.g., virtual queue pair QP3 1554.

The physical queue pair QP 1560 may be reused for this new connectionbecause it has not been placed in a time-wait state. The physical queuepair QP 1560 is then associated with the virtual queue pair QP 3 1554for this new connection. Thus, a new connection is established thatconnects virtual queue pair QP3 1554 on Node l with physical queue pairQP 5 1580 on Node 2 1520. In both the new and the old connection thephysical queue pair QP 1560 is used by Node 1 1510. Therefore, thisoptimizes the use of the hardware resources on the CA of Node 1 1510.

FIG. 16 is a flowchart outlining an exemplary operation of the presentinvention. As shown in FIG. 16, the operation starts with receiving arequest to establish a new connection (step 1610). If this request isfrom another node that is attempting to initiate a connection, therequest may be a REQ message.

A next virtual queue pair from the virtual queue pair pool is selectedfor the new connection (step 1620). A determination is made as towhether this virtual queue pair is in a time-wait state or is alreadyallocated to a connection (step 1630). If so, the operation returns tostep 1620 and the next virtual queue pair in the pool is allocated.

If the virtual queue pair is not in a time-wait state and is not alreadyallocated to a connection, the virtual queue pair is allocated to thenew connection (step 1640). A message is then sent to the other nodewith which a communication connection is to be established (step 1650).If the present node is the initiator, this message may be a REQ message.If the present node is a receiver of a REQ message from an initiatingnode, the message may be a REP message.

Thereafter, assuming that if the present node is the initiator a RTUmessage is received from the other node, communication between the twonodes may begin (step 1660). The operation then ends.

The preceding description illustrates how virtualized QPs can be used tooptimize the use of hardware resources, when the transport type isreliable connected or unreliable connected, a similar technique can beused to virtualize end-to-end contexts and thus, save hardware resourcesassociated with an end-to-end context when using the Reliable Datagramtransport type.

FIG. 17 illustrates the manner by which the present invention may beapplied to end-to-end contexts (EECs). As shown in FIG. 17, similar tothe virtual queue pairs in the embodiments described above, a pool ofvirtual EECs is provided which may be allocated to connections betweennode 1 and node 2. These virtual EECs receive communications via thephysical QP of the node and the physical EECs, as shown. In the same waythat the virtual QPs are placed in a time-wait state when a connectionis torn down between node 1 and node 2, the virtual EECs are placed in atime-wait state when the connection between nodes 1 and 2 is torn down.This enables the physical EECs to be immediately reallocated to anotherconnection using a new virtual EEC, e.g., EEC2 in FIG. 17.

Thus, the present invention provides a virtualized queue pair pool thatmay be used in managing connections between physical queue pairs. By useof the virtualized queue pairs of the present invention, the problemsassociated with processes waiting for a time-wait period to expire areavoided. That is, without this invention, new connection requests mayneed to be rejected until one or more time-wait periods have elapsed tomake the hardware resources available. In large fabrics the time-waitperiod may be quite long. However, with the present invention, thehardware resources are made available immediately by virtue of thevirtualized queue pairs being placed in the time-wait state.

It is important to note that while the present invention has beendescribed in the context of a fully functioning data processing system,those of ordinary skill in the art will appreciate that the processes ofthe present invention are capable of being distributed in the form of acomputer readable medium of instructions and a variety of forms and thatthe present invention applies equally regardless of the particular typeof signal bearing media actually used to carry out the distribution.Examples of computer readable media include recordable-type media suchas a floppy disk, a hard disk drive, a RAM, and CD-ROMs andtransmission-type media such as digital and analog communications links.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain theprinciples of the invention, the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method of establishing a connection between a first node and asecond node in a system area network, comprising: allocating a virtualconnection unit pair to the connection, the virtual connection unit pairbeing associated with the first node; establishing the connectionbetween the virtual connection unit pair of the first node and aconnection unit pair of the second node; and transmitting one or moremessages between the first node and the second node over the connectionusing the virtual connection unit pair, wherein the virtual connectionunit pair has only a virtual connection unit pair identifier and anavailability bit.
 2. A method of establishing a connection between afirst node and a second node in a system area network, comprising:allocating a virtual connection unit pair to the connection, the virtualconnection unit pair being associated with the first node; associatingthe virtual connection unit pair with a physical connection unit pairthat is used for transporting data using the connection, the physicalconnection unit pair being associated with the first node; establishingthe connection between the virtual connection unit pair of the firstnode and a connection unit pair of the second node; transmitting one ormore messages between the first node and the second node over theconnection using the virtual connection unit pair; and tearing down theconnection between the first node and the second node, wherein tearingdown the connection includes placing the virtual connection unit pair ina time-wait state, wherein the physical connection unit pair associatedwith the first node is not placed in a time-wait state.
 3. A method ofestablishing a connection between a first node and a second node in asystem area network, comprising: allocating a virtual connection unitpair to the connection, the virtual connection unit pair beingassociated with the first node; associating the virtual connection unitpair with a physical connection unit pair that is used for transportingdata using the connection, the physical connection, unit pair beingassociated with the first node; establishing the connection between thevirtual connection unit pair of the first node and a connection unitpair of the second node; transmitting one or more messages between thefirst node and the second node over the connection using the virtualconnection unit pair; and tearing down the connection between the firstnode and the second node, wherein tearing down the connection includesplacing the virtual connection unit pair in a time-wait state, wherein aphysical connection unit pair associated with the first node is used toestablish another connection while the virtual connection unit pair isin the time-wait state.
 4. A computer program product in a computerreadable medium for establishing a connection between a first node and asecond node in a system area network, comprising: first instructions forallocating a virtual connection unit pair to the connection, the virtualconnection unit pair being associated with the first node; secondinstructions for establishing the connection between the virtualconnection unit pair of the first node and a connection unit pair of thesecond node; and third instructions for transmitting one or moremessages between the first node and the second node over the connectionusing the virtual connection unit pair, wherein the virtual connectionunit pair has only a virtual connection unit pair identifier and anavailability bit.
 5. A computer program product in a computer readablemedium for establishing a connection between a first node and a secondnode in a system area network comprising: first instructions forallocating a virtual connection unit pair to the connection, the virtualconnection unit pair being associated with the first node; secondinstructions for associating the virtual connection unit pair with aphysical connection unit pair that is used for transporting data usingthe connection, the physical connection unit pair being associated withthe first node; third instructions for establishing the connectionbetween the virtual connection unit pair of the first node and aconnection unit pair of the second node; fourth instructions fortransmitting one or more messages between the first node and the secondnode over the connection using the virtual connection unit pair; andfifth instructions for tearing down the connection between the firstnode and the second node, wherein the fifth instructions for tearingdown the connection include instructions for placing the virtualconnection unit pair in a time-wait state, wherein the physicalconnection unit pair associated with the first node is not placed in atime-wait state.
 6. A computer program product in a computer readablemedium for establishing a connection between a first node and a secondnode in a system area network, comprising: first instructions forallocating a virtual connection unit pair to the connection, the virtualconnection unit pair being associated with the first node; secondinstructions for associating the virtual connection unit pair with aphysical connection unit pair that is used for transporting data usingthe connection, the physical connection unit pair being associated withthe first node; third instructions for establishing the connectionbetween the virtual connection unit pair of the first node and aconnection unit pair of the second node; fourth instructions fortransmitting one or more messages between the first node and the secondnode over the connection using the virtual connection unit pair; andfifth instructions for tearing down the connection between the firstnode and the second node, wherein the fifth instructions for tearingdown the connection include instructions for placing the virtualconnection unit pair in a time-wait state, wherein a physical connectionunit pair associated with the first node is used to establish anotherconnection while the virtual connection unit pair is in the time-waitstate.
 7. An apparatus for establishing a connection between a firstnode and a second node in a system area network, comprising: means forallocating a virtual connection unit pair to the connection, the virtualconnection unit pair being associated with the first node; means forestablishing the connection between the virtual connection unit pair ofthe first node and a connection unit pair of the second node; and meansfor transmitting one or more messages between the first node and thesecond node over the connection using the virtual connection unit pair,wherein the virtual connection unit pair has only a virtual connectionunit pair identifier and an availability bit.
 8. An apparatus forestablishing a connection between a first node and a second node in asystem area network, comprising: means for allocating a virtualconnection unit pair to the connection, the virtual connection unit pairbeing associated with the first node; means for associating the virtualconnection unit pair with a physical connection unit pair that is usedfor transporting data using the connection, the physical connection unitpair being associated with the first node; means for establishing theconnection between the virtual connection unit pair of the first nodeand a connection unit pair of the second node; means for transmittingone or more messages between the first node and the second node over theconnection using the virtual connection unit pair; and means for tearingdown the connection between the first node and the second node, whereinthe means for tearing the connection includes means for placing thevirtual connection unit pair in a time-wait state, wherein the physicalconnection unit pair associated with the first node is not placed in atime-wait state.
 9. An apparatus for establishing a connection between afirst node and a second node in a system area network, comprising: meansfor allocating a virtual connection unit pair to the connection, thevirtual connection unit pair being associated with the first node; meansfor associating the virtual connection unit pair with a physicalconnection unit pair that is used for transporting data using theconnection, the physical connection unit being associated with the firstnode; means for establishing the connection between the virtualconnection unit pair of the first node and a connection unit pair of thesecond node; means for transmitting one or more messages between thefirst node and the second node over the connection using the virtualconnection unit pair; and means for tearing down the connection betweenthe first node and the second node, wherein the means for tearing downthe connection includes means for placing the virtual connection unitpair in a time-wait state, wherein a physical connection unit pairassociated with the first node is used to establish another connectionwhile the virtual connection unit pair is in the time-wait state.